Patrick Baillot

Light types for polynomial time computation in lambda calculus

We present a polymorphic type system for lambda calculus ensuring that well-typed programs can be executed in polynomial time: dual light affine logic (DLAL). DLAL has a simple type language with a linear and an intuitionistic type arrow, and one modality. It corresponds to a fragment of light affine logic (LAL). We show that contrarily to LAL, DLAL ensures good properties on lambda-terms (and not only on proof-nets): subject reduction is satisfied and a well-typed term admits a polynomial bound on the length of any of its beta reduction sequences. We also give a translation of LAL into DLAL and deduce from it that all polynomial time functions can be represented in DLAL.

Elementary Complexity and Geometry of Interaction

We introduce a geometry of interaction model given by an algebra of clauses equipped with resolution (following [10]) into which proofs of Elementary Linear Logic can be interpreted. In order to extend geometry of interaction computation (the so called execution formula) to a wider class of programs in the algebra than just those coming from proofs, we define a variant of execution (called weak execution). Its application to any program of clauses is shown to terminate with a bound on the number of steps which is elementary in the size of the program. We establish that weak execution coincides with standard execution on programs coming from proofs.

Believe it or not, AJM's games model is a model of classical linear logic

A general category of games is constructed. A subcategory of saturated strategies, closed under all possible codings in copy games, is shown to model reduction in classical linear logic.

Light types for polynomial time computation in lambda-calculus

We propose a new type system for lambda-calculus ensuring that well-typed programs can be executed in polynomial time: dual light affine logic (DIAL). DIAL has a simple type language with a linear and an intuitionistic type arrow, and one modality. It corresponds to a fragment of light affine logic (LAL). We show that contrarily to LAL, DIAL ensures good properties on lambda-terms: subject reduction is satisfied and a well-typed term admits a polynomial bound on the reduction by any strategy. Finally we establish that as LAL, DIAL allows to represent all polytime functions.

A polytime functional language from light linear logic

We introduce a typed functional programming language LPL (acronym for Light linear Programming Language) in which all valid programs run in polynomial time, and which is complete for polynomial time functions. LPL is based on lambda-calculus, with constructors for algebraic data-types, pattern matching and recursive definitions, and thus allows for a natural programming style. The validity of LPL programs is checked through typing and a syntactic criterion on recursive definitions. The higher order type system is designed from the ideas of Light linear logic: stratification, to control recursive calls, and weak exponential connectives §, !, to control duplication of arguments.

Linear logic by levels and bounded time complexity

We give a new characterization of elementary and deterministic polynomial time computation in linear logic through the proofs-as-programs correspondence. Girards seminal results, concerning elementary and light linear logic, achieve this characterization by enforcing a stratification principle on proofs, using the notion of depth in proof nets. Here, we propose a more general form of stratification, based on inducing levels in proof nets by means of indices, which allows us to extend Girards systems while keeping the same complexity properties. In particular, it turns out that Girards systems can be recovered by forcing depth and level to coincide. A consequence of the higher flexibility of levels with respect to depth is the absence of boxes for handling the paragraph modality. We use this fact to propose a variant of our polytime system in which the paragraph modality is only allowed on atoms, and which may thus serve as a basis for developing lambda-calculus type assignment systems with more efficient typing algorithms than existing ones.

A feasible algorithm for typing in elementary affine logic

We give a new type inference algorithm for typing lambda-terms in Elementary Affine Logic (EAL), which is motivated by applications to complexity and optimal reduction. Following previous references on this topic, the variant of EAL type system we consider (denoted EAL*) is a variant where sharing is restricted to variables and without polymorphism. Our algorithm improves over the ones already known in that it offers a better complexity bound: if a simple type derivation for the term t is given our algorithm performs EAL* type inference in polynomial time in the size of the derivation.

On light logics, uniform encodings and polynomial time

Light affine logic is a variant of linear logic with a polynomial cut-elimination procedure. We study the extensional expressive power of light affine logic with respect to a general notion of encoding of functions in the setting of the Curry–Howard correspondence. We consider light affine logic with both fixpoints of formulae and second-order quantifiers, and analyse the properties of polytime soundness and polytime completeness for various fragments of this system. In particular, we show that the implicative propositional fragment is not polytime complete if we place some reasonable conditions on the encodings. Following previous work, we show that second order leads to polytime unsoundness. We then introduce simple constraints on second-order quantification and fixpoints, and prove that the fragments obtained are polytime sound and complete.

Checking Polynomial Time Complexity with Types

Light Affine Logic (LAL) is a logical system due to Girard and Asperti offering a polynomial time cut-elimination procedure. It can be used as a type system for lambda-calculus, ensuring a well-typed program has a polynomial time bound on any input. Types use modalities meant to control duplication.

Verification of Ptime Reducibility for system F Terms: Type Inference in Dual Light Affine Logic

In a previous work Baillot and Terui introduced Dual light affine logic (DLAL) as a variant of Light linear logic suitable for guaranteeing complexity properties on lambda calculus terms: all typable terms can be evaluated in polynomial time by beta reduction and all Ptime functions can be represented. In the present work we address the problem of typing lambda-terms in second-order DLAL. For that we give a procedure which, starting with a term typed in system F, determines whether it is typable in DLAL and outputs a concrete typing if there exists any. We show that our procedure can be run in time polynomial in the size of the original Church typed system F term.